This application claims benefit of priority under 35 USC xc2xa7 119 to Japanese Patent Application No.P2000-153057, filed on May 24, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an inspection method of detecting defects on an array substrate that is mainly used in a liquid crystal display (LCD) device of active-matrix type, and, more particularly, to an inspection method of detecting defects in analogue switches, each made up of TFT, formed on an array substrate.
2. Description of the Related Art
Recently, liquid crystal display (LCD) devices of active-matrix (AM) type have become widely available as the display devices for notebook type personal computers and mobile type information terminals. In the LCD device of AM type, a switch element is placed for each picture element. In particularly, use of the display devices made up of poly-silicon thin film transistors (TFT) can contribute to performing easy wiring and also reduce the size of devices because driver circuits in addition to an element section can also be integrated on an array substrate.
The driver circuits integrated on the array substrate include signal line drivers. Each signal line driver comprises a shift register, a control circuit, a buffer circuit, an analogue switch ASW, and a video bus. This ASW comprises a CMOS transistor that is formed by a combination of n-channel TFT and p-channel TFT because it samples image signals of different polarity on a signal line per horizontal scanning period or per frame period in order to reverse its polarity.
By the way, a small amount of a current flows from a drain to a source of such a TFT even if it is in OFF state (namely, a non-conductive state). Hereinafter, the small amount of the current is referred to as an off-leak current. Under a normal frame frequency, the off-leak current does not affect any display image because the writing for a following image signal is performed before the voltage of the image signal that has already been written into the picture elements is reduced by the off-leak current.
However, when the amount of the off-leak current is increased by the variations of the characteristics of transistors such as TFT, the voltage of the image signal written into the picture elements is greatly fallen during one frame period. This cannot maintain the voltage level that is necessary to display the image signal on the LCD device (hereinafter, this state will be referred to as xe2x80x9coff-leak defectsxe2x80x9d).
As a result, when a black (or another color corresponding to this black) is displayed on the LCD device, a color of picture elements in a column to which the image signal of the black has been written through the ASW in the off-leak defect becomes light, for example. The user recognizes this state as a line defect.
Such the defect above can be detected by performing a lighting test after the LCD panel is manufactured. However, in this state, since many LCD panels are conveyed on a manufacturing line, many LCD panels in defect can be manufactured and this causes to reduce the manufacture yields. Furthermore, because of the waste of the processing time and the parts to manufacture the LCD panels, the manufacturing costs for the LCD panels become increased.
Thus, it is preferable to detect the line defect on a LCD panel caused by the off-leak defect during the array substrate fabrication process as early as possible because it is difficult to improve the line defect caused by the off-leak defect after the completion of the manufacturing of the LCD panel.
In order to detect the defect on the array substrate fabrication process, an array tester is currently used. This array tester can perform the inspection that is equivalent to perform the inspection of displaying actual image on the LCD panel after manufactured.
In addition to the off-leak defect of ASW, the array tester can inspect a line defect, a point defect of the picture element, the amount of a current flowing through a scan line/signal line driver circuit, and the operation of a shift register as driver detection.
The ASW off-leak judgment method of comparing a signal wave with an expected value of a signal wave is well known. This signal wave is obtained by writing a test signal into a supplemental capacity connected to each picture element, and by reading this test signal after the writing for one frame has been completed.
However, because the amount of a leak current caused by the off-leak of ASW is little, a small difference of the waveform of the test signal is buried in a measurement error and a noise. Hence, it is difficult to detect the difference of both the waveforms of the test signal and the expected value. Further, even if the judgment for the defect is performed correctly, it is difficult to distinguish the cause of the defect, for example, it is caused by the off-leak defect, or by a short in a wiring, or by other kind of defect.
Thus, the conventional inspection method using the array tester cannot distinguish the kinds of defects, that is, whether or not the detected defect is caused by the off-leak defect of ASW or by the line defect during the array substrate fabrication stage, and it is also difficult to take an optimum countermeasure before the LCD panels including the line defect are conveyed on the manufacturing line and it cannot be avoided to increase the manufacturing costs.
Accordingly, an object of the present invention is, with due consideration to the drawbacks of the conventional technique, to provide an array substrate inspection method of easily detecting the off-leak defect of ASW during the array substrate fabrication process.
According to an aspect of the present invention, the array substrate inspection method can be applied to an array substrate. For example, the array substrate comprises: a plurality of signal lines; a plurality of scan lines intersecting with the plurality of signal lines; a picture element electrode formed on each intersection of the signal lines and the scan lines; a supplemental capacitor electrically connected to each picture element electrode; a switching element through which the signal line is electrically connected to the corresponding picture element electrode in order to write an image signal supplied through the corresponding signal line to the picture element electrode and the supplemental capacitor based on a gate signal supplied through the corresponding scan line; a video bus through which the image signal is transferred; a signal line driving circuit having analogue switches (ASWs) and a control circuit to control the ON/OFF operation of the ASWs, each ASW supplying the image signal on the video bus to the signal line by electrically conducting the video bus to the corresponding signal line; and a scan line driving circuit supplying the gate signal. In the array substrate inspection method of an embodiment of the present invention, an inspection step is repeated a desired number of times. In each inspection step, a test signal (namely, an image signal to be used in test) supplied to the video bus is written to the supplemental capacitor by entering the analogue switch into a conductive state based on the selection signal, the test signal written in the supplemental capacitor is stored during a desired time period by entering the analogue switch into a non-conductive state by a non-selection of a desired voltage which is different from each other in each inspection step, and the test signal is read from the supplemental capacitor through the corresponding signal line. In each inspection step, the value of the voltage of each non-selection signal to be supplied to the analogue switch is changed.
Thus, in the array substrate inspection method, when the voltage of the non-selection signal (namely, the off-voltage Vgs by which the analogue switch (ASW) enters OFF) is shifted to another voltage, the amount of the leak-current flowing between the drain and the source thereof during the off-state of the analogue switch ASW is also shifted. For example, when a characteristic defect occurs in a p-channel TFT and the off-voltage Vgs forming the ASW is zero, the amount of the leak current is almost equal, namely not changed in both the normal state and the off-leak defect state in the ASW. On the contrary, when the off-voltage Vgs=xe2x88x921 Volt, the amount of the leak current in the ASW is greatly changed in both the states. That is, when the off-leak occurs in the ASW, the amount of the leak current becomes large when the off-voltage Vgs=xe2x88x921 Volt. It is thereby possible to easily detect the defect of the array substrate even if the waveform of the test signal includes noise. Further, it is also possible to easily distinguish the off-leak defect from a defect of another type based on the difference of the magnitudes of the two test signals. By the way, the setting of the voltage of the off-voltage for easy evaluation of the waveform of the test signal is changed according to the kind, the degree, and the location of the defect.